This invention relates generally to the treatment or surface modification of a metal component by diffusion alloying, in order to increase its hardness and resistance to wear by abrasion, and its corrosion-resistance. More particularly, the invention relates to such treatment or surface modification of only a portion of a metal component, such as a portion of a steel boiler tube or other elongated component.
Chromizing is a thermally activated diffusion process that is used to produce a high chromium-content surface layer on an iron or steel surface. This process is used on boiler tubes, pipes and other metallic components of chemical recovery boilers, coal-fired utility boilers and other types of industrial equipment to provide a surface which is resistant to erosion, abrasion, oxidation and corrosion. Iron and steel components such as boiler components are often chromized by a process known as pack diffusion, wherein a pack mixture comprising chromium or ferrochromium, an inert filler such as alumina (Al2O3), and a halide activator such as ammonium chloride (NH4Cl), are blended together. The component is then placed in the pack mixture in a retort having an atmosphere which is controlled so as to preclude oxidation. If it is desired to chromize only the internal surface of a boiler tube, the tube itself may be filled with the pack mixture and a cap welded into place on each end of the tube, so that the tube itself becomes a self-contained retort. The retort is then heated to an elevated temperature for a specified period of time. A typical pack diffusion thermal cycle involves holding the retort and its contents at a temperature within the range of 1800xc2x0-2000xc2x0 F. for one to ten hours. This heating causes the chromium in the powder to gasify, to deposit on the boiler component and to diffuse into the base metal of the boiler component. In such diffusion process, the chromium atoms physically and metallurgically penetrate the base metal surface of the component and substitute for some of the iron atoms of the base metal. This diffusion process results in an iron or steel boiler component having an iron-chromium alloy coating that is metallurgically bonded as an integral part of the base metal of the component. Since the structural modifications to the boiler component which result from the diffusion process occur within the surface of the base metal and not on the surface itself, the diffused chromium is an integral part of the surface of the base metal which is not subject to the spalling or peeling that may characterize mechanically bonded coatings.
Similar processes may be used to diffuse other elements, such as aluminum, nickel, silicon, boron or zinc, into the surface of a metal component. Specific chromizing and other diffusion processes are described in U.S. Pat. No. 2,825,658 of Samuel, U.S. Pat. No. 3,622,402 of Baranow et al., U.S. Pat. No. 3,785,854, U.S. Pat. No. 3,801,357, U.S. Pat. No. 3,958,046, U.S. Pat. No. 4,290,391, U.S. Pat. No. 4,350,719, U.S. Pat. No. 4,694,036, U.S. Pat. No. 4,820,362, U.S. Pat. No. 4,830,931 and U.S. Pat. No. 5,194,219 of Baldi, U.S. Pat. No. 4,469,532 of Nicolas, U.S. Pat. No. 4,485,148 of Rashid et al., U.S. Pat. No. 4,904,501 and U.S. Pat. No. 5,041,309 of Davis, U.S. Pat. No. 4,963,395 and U.S. Pat. No. 4,993,359 of Lewis et al., U.S. Pat. No. 5,135,777 and U.S. Pat. No. 5,208,071 of Davis et al., and U.S. Pat. No. 5,582,867, U.S. Pat. No. 5,672,387, U.S. Pat. No. 5,747,112 and U.S. Pat. No. 5,803,991 of Tsubouchi et al.
Although these patents describe various methods for diffusing various elements into the surface of a metal component, most such methods require that the entire component be placed into a sealed retort for such treatment. Those which describe the treatment by diffusion alloying of only a part of a metal component require that the portion of the component that is not to be treated must be masked prior to placing the entire component in a retort. Care must be taken to completely mask the portion that does not require treatment and to avoid scratching such masked portion to insure that masking is effective. Since masking methods are inconvenient at best and sometimes ineffective as well, it would be desirable if a method could be developed by which a portion of a metal component could be treated by diffusion alloying without requiring masking or other special treatment of the remainder of the component.
Among the advantages of the invention is the fact that it permits the diffusion alloying of only a portion of a metal component without requiring that the part of the component not to be treated be masked. Another advantage of the invention is that its ease of operation and low cost renders diffusion alloying of only a portion of a component convenient, thereby avoiding the situation where entire components are treated by diffusion alloying when only a portion thereof is required to be so treated. Still another advantage of the invention is that it does not require that an inert gas atmosphere or flow be supplied in the diffusion process.
Another advantage of a preferred embodiment of the invention is that it facilitates diffusion alloying at a rate considerably more rapid than is disclosed in the prior art.
Additional objects and advantages of this invention will become apparent from an examination of the drawings and the ensuing description.
As used herein, the term diffusion alloying refers to a thermally activated diffusion process by which a surface layer containing an alloying element is produced on a metal component.
As used herein, the term ferrous alloy refers to an alloy that is at least 50% by weight iron.
The invention comprises a method for treating a portion of a metal component by diffusion alloying. According to this method, a container with at least one open end is provided. The container also has a width that is greater than the width of the portion of the metal component to be treated and a depth that is greater than the length of the portion of the metal component to be treated. The portion of the metal component to be treated is placed in the container, and a heat-activated alloying powder is placed in the container around the portion of the component to be treated in a layer that extends along the length of the portion of the component to be treated. A non-oxidizing powder is also placed in the container adjacent to the alloying powder and around the metal component in a layer that extends to an open end of the container. A cap is provided for each open end of the container, which cap is adapted to seal the container around the metal component except for gases which are produced in diffusion alloying. A furnace that is adapted to heat the portion of the metal component to be treated to activate the alloying powder is also provided. The container with the portion of the metal component to be treated therein is placed in the furnace, and the furnace is operated to heat the portion of the metal component in the container to a temperature and for a time sufficient to cause diffusion alloying of the portion of the metal component to be treated by the alloying powder.